The addition of a flange does not improve the pressure generated during cemented acetabular cup implantation

Abstract Flanged acetabular cups were developed with the rationale that, at insertion, they would increase the pressure of the cement and improve penetration of cement into the acetabular bone. Various studies have been inconclusive regarding their effectiveness. In this work, we aimed to eliminate all confounding factors and measure the pressures generated during acetabular pressurization and cup implantation using a simplified steel acetabulum, high precision pressure transducers, proper surgical techniques and two acetabular cups, identical apart from the addition of a flange to one. It was found that the flanged acetabular component did not significantly increase the pressure in the acetabulum and in some cases reduced the pressures generated when compared to an unflanged cup. The addition of a flange did not reduce the pressure differential between the pole and the rim of the acetabulum, nor did it have a significant effect on pressure lost over the cup implantation period. It was concluded that flanged acetabular cups provide no significant improvement in the pressures generated in the acetabulum during acetabular cup implantation. It is hypothesized that the flange may be seen as a design feature intended to slow the insertion of the cup into the cement, thus requiring the surgeon to apply a larger load in order to correctly position the acetabular cup; in this way larger pressure will be generated.

Of all failed THRs, many fail due to aseptic loosening of the cup. 2,4 Early radiolucent lines on the radiograph between the cement and the bone, particularly progressive ones, are a reliable predictor of later loosening. [5][6][7][8] The reamed acetabulum is a shallow cavity with a large, irregular opening; this makes it difficult to maintain a high pressure at the bone surface both during pressurization and cup insertion.
The addition of a flange to the acetabular cup was claimed to improve pressurization, prevent the acetabular cup bottoming out (where the cup makes contact with the bone, thus stopping further pressurization) and to minimize cup movement during implantation. 9 A flange was proposed to provide uniform pressurization and thereby optimize cement intrusion to the subchondral bone which has been showed to improve the interface strength. 10 The literature on this topic contains limited data regarding the pressures generated at the acetabulum surface and there is contradictory experimental evidence regarding the efficacy of flanged acetabular components. [11][12][13][14][15][16][17] Therefore the aim of this study was to contribute to the existing literature regarding whether the addition of a flange to the   acetabular component alters the cement pressure distribution at the   surface of a model acetabulum during cup implantation. It also aimed to   provide a detailed pressure profile at the acetabulum surface which all other similar studies fail to provide.
Three key questions were asked: Omega PX61V0 pressure transducers were used with an Omega TXDIN1600S bridge for amplification and data acquisition. The pressure sensors were calibrated using a loading program and a doughy F I G U R E 1 A radiograph of a cemented THR with annotations. 1 substance which produced known pressures at each position of the acetabulum; this was done prior to each experiment session. The transducers were made flush to the acetabulum hemispherical surface using shim washers. The data was filtered using a first order, low pass Butterworth filter with a cut-off frequency of 0.00625 Hz, selected using the Nyquist criterion.
A Type K thermocouple was used to monitor the temperature, as temperature is often used to monitor the progress of polymerization.
The thermocouple was inserted into the acetabulum cavity between the acetabular rim and the pressuriser. The location of the tip of the thermocouple was not controlled; therefore, the magnitude of the temperatures measured cannot be directly compared between tests but the data can still be used to calculate the cure-time which is defined as the time at which the cement was halfway between the ambient temperature and maximum temperature reached. 19 CMW 2 (Depuy Synthes); a high-viscosity cement frequently used for fixation of the acetabular component was used to implant the acetabular cups. For each experiment two 20 g packets of cement were used as this sufficiently filled the cavity. The cement used has been subject to many previous studies.
The assembled rig was mounted into a Shimadzu ADS-X which was used to apply load. It was fitted with a 1 kN load cell ( Figure 4). Note that during surgery, the cup is implanted at 40 to the transverse plane, however, the force applied by the surgeon is orthogonal to the plane of the cup face, therefore the experimental set up here is the same.
All equipment used was manufactured with a tolerance of ± 0.05 mm, with consideration of the design of the rig, the loading was always applied within 0.25 mm from the center of the acetabulum cavity.

| METHODOLOGY
The methodology employed here is identical to that of a previously published study. 20 The temperature of the laboratory was between 20.5 C and 23 C for all experiments. The humidity of the lab was between 45% and 50%. All equipment was left in the lab to ensure that the temperature of the equipment was static. 19 Mold release spray (Silicone Mold Release Agent, Ambersil) was used to ensure that the cement mantle could be removed from the model acetabulum.
The Shimadzu was force controlled with a maximum stroke rate of for acetabular pressurization. 21 The pressures generated in an experiment by New et al. resulted in similar pressures at the acetabulum as was found in preliminary experiments using a force of 100 N. 22 Parsch et al. used a force of 60 N for pressurization. 18 Beverland et al. used a 10 kg mass to apply force during pressurization. 15 A load of 100 N was used as it reflected the previous literature and the opinion of the surgeons co-authoring this paper. The timing was determined so that pressurization and cup insertion would both be completed within the working phase of the cement.
The pressuriser was then removed from the Shimadzu and a cup was placed into the cement. The cup implantation program was started, a load of 50 N was applied until the cement was fully cured ( Figure 6). After the cement had fully cured, the cement mantle was removed, and another test was performed. This was repeated five times for each of the four testing conditions: two cup designs and two mixing methodologies. F I G U R E 5 A force was applied to bone cement in acetabulum using 100 N force on Depuy pressuriser.
F I G U R E 6 Pressure applied to the acetabular cup using a 50 N force.
The end of cup implantation was taken to be when there was a significant deviation from the average pressure. To allow a more detailed analysis of the continuous pressure curves they were divided into fifths and the pressure at each of these five points in time was taken and used for statistical comparisons. (Figure 7). This technique also allowed for analysis of how the pressure evolves, previous studies often only state the average or maximum pressure achieved during surgery, but this is not enough information for proper analysis.
A Ryan-Joiner test was used to test for normality, if p ≤ 0.05 it was concluded that the pressure data at that time was not normally distributed. A student t-test was used if both sets of data being compared were found to be normally distributed. If one or both sets of data were found to be non-normal, then a Mann-Whitney test was used. A difference in the means was considered significant if p ≤ 0.05. This statistical methodology was used for all comparisons of data. The spike in pressure between pressurization and cup insertion is caused by the placement of the acetabular cup into the cement.

| RESULTS
The averages and the standard deviations of the cement pressure for each pentile (fifth) of cup insertion, at each angle, for each condition can be seen in Table 1. The equivalent data for pressurization is not provided as no statistical difference was found in the pressures generated due to which cup was subsequently implanted (p < 0.05) and no significant change in pressure was observed through time (p < 0.05). With the exception of the first and second pentile of cup insertion for an unflanged cup with vacuum mixed cement, there was always a significant difference between the pressure generated at the rim and the pole of the acetabulum during cup insertion (Figure 9).
There was no statistically significant drop in pressure for any set of data.
Upon closer inspection of the removed cement mantles, it was found that none of the cups bottomed out and the cement mantle was thicker than 2 mm for all repeats at all angles from the direction of loading.

| DISCUSSION
The aim of this study was to answer three questions regarding cement behavior in the acetabulum during implantation of unflanged and flanged acetabular cups. Firstly, it was found that the addition of a flange to the acetabular cup did not increase the pressure generated in the cement at the acetabulum bone surface during cup implantation. There were only three significant differences found in pressure due to the cup design; the unflanged cup generated larger pressures. Secondly, except for the first two pentiles of vacuum mixed, unflanged cup insertion; it was found that the pressure was always significantly larger at the pole of the acetabulum (0 ) than at the rim (75 ) during cup implantation; there is no evidence to suggest that the addition of a flange significantly reduced the pressure differential. Finally, it was found that there was no decrease in pressure over time for any of the testing conditions. A good bond between the bone cement and bone is a key for the longevity of total hip arthroplasty implants as more interdigitation increases the contact area between cement and bone and thus decreases contact stresses. 10 Suboptimal bonding can be observed on postoperative radiographs as a radiolucent line between the cement and the bone. These are most frequently observed near the rim of the interface. 24 It has been shown that the penetration depth of bone cement into the bone is dependent on the pressure generated during implantation. 25 The strength of the cement-bone interface is dependent on the depth of penetration. 10,26 Therefore, it is key that the cement pressure generated during pressurization and cup insertion should be uniform and sufficiently large to achieve optimal penetration across the acetabulum.   Note: Statistical differences between flanged and unflanged cups are highlighted using a, b, and c to indicate the relevant pair.

| Pressurization
In 1999, New et al. measured pressures generated in vivo during pressurization and found values of 49 ± 17 kPa and 47 ± 17 kPa for two surgeons. 22 The results reported in our study are within that range.
Noble and Swarts found that the cement pressure required to achieve an optimal cement penetration of 3-5 mm varied significantly with cement brand and bone porosity and therefore there is not an ideal pressure to aim for. 27 Although it appears pressures were measured at the rim and the pole in a study by Bernowski et al. they do not report figures for the "sustained pressure" but only provide the peak pressure at the rim. Estimating from a provided chart it appears that the sustained pressure at the rim was between 80 and 90 kPa and between 60 and 80 kPa at the pole. This is for an applied load of 201 N. This finding is not reflected in our results where the pressures generated were larger at the pole than the rim. If full contact is assumed between the cup and cement and the difference in the applied load is accounted for, then the magnitude of the pressure appears to be similar to our study. In a chapter on "optimal cementing technique," Parsch et al. published a graph that report the pressures generated across the acetabular surface using a standard acetabular pressuriser. Although the peak pressures generated were larger than in our study (≈130 kPa), they found no pressure differential during pressurization as was also found in our study. 28  The pressuriser effectively seals the acetabulum cavity, and the viscosity of the cement is still sufficiently small so that the pressure is equalized. At the cup implantation stage, there is a flow of excess cement that must be displaced for correct cup positioning. This flow must be driven by a pressure differential. This flow at the cup implantation stage while the cement is curing may be of importance.

| Cup insertion
In this study, it was found that the pressure at the pole (0 ) in the fourth pentile of cup insertion for non-vacuum mixed cement was larger for unflanged cups, generating a pressure 14.7% larger than flanged cups. The other location of significant difference between the pressures generated was for the last two pentiles of vacuum mixed cup insertion at 45 from the direction of the applied loading. The study reported here is novel in that the methodology included pressurization of the cement prior to cup insertion, thus F I G U R E 9 Boxplots of the average pressure for the unflanged, vacuum mixed condition. Each pair of boxplots represents the pressure at 0 and 75 for each pentile of cup insertion more closely simulating an in vivo implantation. It is also novel as the whole pressure profile through time was recorded and is reported here, allowing future researchers to refer to this study when a methodology is being designed. Preliminary testing was performed to ensure that the forcing program would not cause "bottoming out" or This explanation is a key outcome of this study.
With some exceptions stated above, this study found that for the most testing conditions, there is always a significant difference between the pressure at the rim and at the pole of the acetabulum during cup insertion ( Table 1). As the acetabular cup is inserted, it will create a pressure differential, driving the flow of cement around the cup and out of the acetabulum. As polymerization continues the viscosity of the cement increases, reducing and eventually stopping the flow of cement out of the acetabulum.
There are significant differences between this experimental in vitro study and the clinical in-vivo setting, however, this study was designed to reduce confounding factors so that concrete conclusions could be drawn. Only one cement was used in this study, more cements should be tested to determine whether the conclusions drawn apply more generally. More work should be done to determine the loading applied in vivo, an instrumented acetabular pressuriser is cur- Further tests should be performed to determine whether the difference between the diameter of the acetabular cup and the acetabulum makes a significant difference to the pressures generated at the acetabulum surface. The rim of an anatomically correct acetabulum is irregular this would probably lead to larger gaps between the pressuriser and the acetabulum and the cup and the acetabulum. Although the cement penetration was not directly measured it has been shown that penetration is improved with an increased pressure. 10  The results suggest that flanged cups provide no advantage in terms of an increase of the pressure differential between rim and pole.
Nor does the addition of a flange increase the pressure magnitude compared to unflanged acetabular cups. The data reported here suggest that unflanged cups may produce a larger cement pressure than flanged cups during acetabular cup insertion for the same insertion load.

| CONCLUSION
The results from this study demonstrate that the flange itself does not increase the pressure of cement in the acetabulum but rather, the increased contact area between cup and cement due to the addition of a flange reduces the pressure generated for the same applied load and slows the insertion of the cup. Therefore, to achieve correct positioning in good time, the surgeon will have to apply a larger load to the flanged cup.

ACKNOWLEDGMENTS
We would like to thank Dr Rob Bigsby for his significant technical advice on cementing techniques. We would also like to acknowledge technical assistance from Stuart Baker for manufacturing all the components and general laboratory assistance from Paul Scott and Chris Brown.

CONFLICT OF INTEREST
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.